Environmental test switch for intruder detection systems

ABSTRACT

A method and apparatus for optimizing the sensitivity of intruder and fire detection systems for a predesired margin of safety against false alarming. The apparatus comprises means for selectively increasing the system sensitivity by a calibrated amount above an optimum operating level, and means for adjusting the selectively increased system sensitivity, to a level at which the system just responds (or just fails to respond) to the background noise level. Thus, when the sensitivity is returned to its normal operating level, a built-in margin of safety is provided against false alarming from noise signals.

United States Patent McMaster Sept. 24, 1974 [54] ENVIRONMENTAL TESTSWITCH FOR 2307,01: 9 195: litmzm L! I 340/26! x [NTRUDER DETECTIONSYSTEMS 3.l()-).l65 lU/lih} Hagnu Mil/25R l) 3,786,468 l/l974 Moffitl34(l/2l4 [75] Inventor: Richard Laverne McMaster,

Rochester Primary Examiner-David L. Trafton [73} Assignee: DetectionSystems, Inc., Fairport, Almmeyl 8 Firmwa"en KUTZ NY. 221 Filed: Feb. 9,1973 [57] ABSTRAC? A method and apparatus for optimizing the sensitivity[21 1 PP 331,202 of intruder and fire detection systems for a predesiredmargin of safety against false alarming. The apparatus 52 Cl 340/214,340/258 R, 340/258 A, comprises means for selectively increasing thesystem 340/26] sensitivity by a calibrated amount above an optimum 51lm. Cl. G08b 29/00 Operating levcl and mans for adjusting theSelectively [58] w f Search H 340/253 R, 253 B 5 A, increased systemsensitivity, to a level at which the 340/214, 410, 261 system justresponds (or just fails to respond) to the background noise level. Thus,when the sensitivity is 5 References Cited returned to its normaloperating level, a built-in mar- UNITED STATES PATENTS gin of safety isprovided against false alarming from noise signals. 2,566,367 9/l95lPeters 340/214 2,655,645 10/1953 Bagno r. 340/258 A X 3 Claims, 3Drawing Figures n a l l GAIN q 20 CONTRQL l /9 I I DOPPLER ALARMRECEIVER 215,1 E/L TER INTEGPATOR RELAY TRANSDUCER 3/ 7 33 34 35 I ITUNED Z7 DETECTOR THRESHOLD l PREAMP AMP DETECTOR i l TEST SWITCH I 8 Td w w "I TRANSMITTER ZZQ OSCILLATOR 1 I0 I TRANSDUCER I l l l lPAIENIEusEPmm amflme.

F I GA/N 20 CONTROL 19 I I DOPPLER ALARM l REE/V5? SIGNAL F/L TERnwmRAroR RELAY ITRANSDUCER SUMMER AMP ru/vm 27 0ErEcr0R THRESHOLD IPREAMP 4MP DETECTOR I 1 40 1 TEST SWITCH R/l/ER TRANSMITTER 3 OSCILLATORI /0 T0 ALARM RELAY FROM F/L TE? F/G3 F-w FREQUENCY ENVIRONMENTAL TESTSWITCH FOR INTRUDER DETECTION SYSTEMS BACKGROUND OF THE INVENTION Thepresent invention relates to intruder and fire detecting systems, and,in particular, to a method and apparatus for optimizing the sensitivityof such systems for a predesired margin of safety against false alarmingdue to spurious sources comprising the background noise.

Proper installation of conventional electronic intruder detectingsystems requires that the installer have some knowledge of thecharacteristics of the background against which the intruder is to bedetected. ln general, he should know the amplitude of the back groundnoise within the bandpass of the receiver component of the system, aswell as the duration and frequency of occurrence of transient signalswhich arise within such bandpass. Having gained this knowledge, theinstaller can then adjust the sensitivity of the system to a level whichprovides optimum protection; i.e., a level which provides a high degreeof sensitivity to the event of interest without rendering the systemprone to false alarming.

The most effective determination of the signal characteristics of thebackground noise is, of course, to monitor the output of the signalprocessing circuitry of the system, with an electronic oscilloscope orsome other electronic test device. Ideally, such monitoring is doneafter the installer has activated all conceivable sources of noise inthe environment wherein the system is situated. This approach, however,suffers the drawback of requiring costly equipment, in addition to acertain amount of skill and judgment on behalf of the installer.Typically, the amplitude of the noise signal fluctuates wildly,requiring the installer to make what amounts to an educated guess as tothe maximum noise level, frequency of occurrence, and duration oftransients which might be encountered during the use of the system. Theproblem with such guessing, of course, is that if the installer guesseswrong, the result is either a high frequency of false alarms, resultingfrom a system sensitivity which is set too high, or inadequateprotection, resulting from a system sensitivity which is set needlesslylow.

SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto take the guesswork out of properly adjusting system sensitivity ininstalling intruder detection and other types of condition-responsivesystems.

Another object of the invention is to provide an improved intruderdetection system in which the system sensitivity can be readilyoptimized for a predefined margin of safety from false alarms withoutthe need for costly electronic test equipment and substantial skill onbehalf of the installer.

These and other objects of the invention are achieved by the provisionof an electronic condition-responsive system which includes in additionto a means for adjusting the system sensitivity, circuit means forincreasing at any time the system sensitivity by a calibrated amount.According to a preferred embodiment, such circuit means, when activated,preforms one or more of the following functions: (1) widens thebandwidth of the bandpass filters of the signal processing circuitry;

(2) reduces the alarm-activating threshhold of such circuitry; (3)reduces the integration time of the integrator component of the signalprocessing circuitry; (4) increases the gain of one or more receiveramplifiers; and (5) increases the output of the transmitter, All of theabove functions are carried out so as to increase the system sensitivityby a known factor, such factor representing a predesired margin ofsafety from responding to false alarm-producing background noise.According to the inventive process, system sensitivity is optimized forthe predesired margin of safety by merely l activating the circuitmeans, thereby increasing the system sensitivity by a known amount; (2)adjusting the system sensitivity to a level which is just high enoughnot to respond to the maximum steady state noise signal in theenvironment where the system is used; and (3) inactivating the circuitmeans to reduce the system sensitivity by said known amount level.

The above objects of the invention, as well as its various advantages,will become immediately apparent to those skilled members of the artfrom the ensuing detailed description, reference being made to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of anultrasonic intruder detection system embodying the invention;

FIG. 2 is an electrical schematic of the integrator and threshholdsensing components of the system illustrated in FIG. 1, together withpreferred circuitry for increasing the sensitivity of such components;and

FIG, 3 illustrates the effect which an adjustment in threshold has onthe bandwidth of the signal processing circuitry.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT In FIG. I, an ultrasonicintruder detection system embodying the invention is illustrated inblock diagram form. Such a system operates on the well known Dopplereffect, being of the type disclosed in my copending application Ser. No.273,472, filed on July 20, 1972 now U.S. Pat. No. 3,803,539. It shouldbe understood from the outset that this particular system merelytypifies the types of detection system in which the invention hasutility.

Ultrasonic energy waves 8 emanating from a transmitter 10 are directedinto a region in which the motion of an intruder is to be detected.Transmitter l0 preferrably comprises a piezoelectric ceramic transducer11 which is driven at an ultrasonic frequency (e.g., 40 KHz), by acrystal-controlled oscillator 12 via driver amplifier 14. The exactfrequency control afforded by the use of a crystal-controlled oscillatorpermits several Doppler systems to operate in the same general areawithout generating undesirable beat frequencies.

Ultrasonic energy waves 19 reflected back toward the receiver 20 aresensed by a receiver transducer 21, also ceramic, which converts theultrasonic energy into an electrical signal having an instantaneousfrequency, phase and amplitude characteristic of the resultantultrasonic energy wave at the receiver transducer 2]. Transducer 21 hasa bandpass characteristic which, in combination with a tunedpreamplifier 23, through which the output of transducer 21 is passed,causes the device to respond only to those signals centered within a fewkilohertz of the transmitter frequency. Such front end tuning enablesthe system to ignore ordinary audible noises.

As long as there is no motion in the region under surveillance, thereceived signal level will be constant, except for slow drifts producedby changes in temperature and humidity which affect the speed of sound,and the frequency will be identical to that of the transmitted energywave. However, in the event an object within said region moves so as tohave a component of motion toward or away from the receiver, sound wavesreflected from it will fluctuate in both amplitude and frequency which,in turn, will produce similar fluctuations in the output of transducer21.

Amplitude changes in the output of preamplifier 23 are sensed bydetector amplifier 25 which is biased such that there is more gain forexcursions of one polarity than for equal excursions of the oppositepolarity. This non-linear operation, together with some built-incapacitance, serves as an envelope detector with gain; the envelopefrequency being, of course, equivalent to the Doppler frequency.

In addition to sensing the Doppler frequency via amplitude changes inthe received signal, the Doppler frequency is also sensed via a signalinjection technique. Prior to being fed to the input of detectoramplifier 25, the output of preamplifier 23 is combined with a largeconstant amplitude signal of the transmitter frequency which is injectedfrom the output of driver amplifier 14. Actually, the injected signal isalgebraically summed with the preamplifier output via summer 27. Whenthe injected signal and received signal are in phase, they reinforceeach other producing an increase in the output from summer 27. When theinjected and received signals are out of phase, a decrease in the summeroutput is produced. This amplitude variation, arising from frequency orphase shifts generated by a moving target, is also detected by detectoramplifier 25 which contains a low pass filter which removes the 40 KHzcarrier frequency, leaving only a signal which fluctuates at a rateequal to target-related Doppler amplitude and frequency changes.

While serving as a reference against which the frequency (or phase) ofthe received signal is compared for the purpose of detecting the Dopplersignal, injection also serves to stabilize the sensitivity of theDoppler device. Preferably, the injected signal is larger than themaximum anticipated output of tuned preamplifier 23. Because of this,the amplitude of the carrier signal remains relatively constant eventhough there is considerable variation in received energy at thereceiver transducer due to phase cancellations. This tends to maintainconstant sensitivity notwithstanding device placement and substantialchanges in environmental conditions. When a null condition exists andthe output of the receiver transducer is substantially zero, theinjected signal serves to overcome forward diode-drop in the detector.The injected signal, therefore, provides a bias to maintain the detectorat uniform sensitivity.

The Doppler signal output of detector amplifier 25 is next fed to theDoppler signal amplifier 29 which comprises a conventional high-gainaudio amplifier preferably having a bandwidth encompassing the Dopplerfrequencies of interest. The gain of amplifier 29 is sufficient toproduce an overdriven squared signal for targets within the sensitivityrange of the device. The output of amplifier 29 is then fed to a filter31, preferably a digital filter circuit such as that disclosed in U.S.

Application Ser. No. 20,887, filed Mar. 19, 1970, to further eliminatesignals uncharacteristic of the motion of interest. After integration ofthe filter output by inte grator 33 to eliminate the effects oftransients, the signal is then used to trip an alarm relay 35 or thelike.

The sensitivity of the above-described system (i.e., its responsivenessto motion) is governed by a variety of parameters including, forinstance, the gain of amplifiers 23, 25 and 29, the intensity oftransmitted waves 8, the bandpass of filter 31, the time constant ofintegrator 33 and the threshhold level of threshhold detector 34. Byadjusting these parameters collectively or separately, the system can behypersensitized to the extent that it will respond to the slightestmovement of a small distant target. On the other hand, the adjustment ofthese same parameters can result in a desensitization of the system tothe extent that it will fail to respond to a moving target, regardlessof its size, rate or direction of movement, or its proximity to thesystem. The optimum values of these parameters commonly depend upon twofactors: (1) the noise level of the environment in which the system isused; and (2) the characteristics of the electrical signals which can beanticipated from the object of interest.

Since the system manufacturer is usually aware of the target signalcharacteristics, most of the parameters can be set at an optimum valuebefore the system is shipped to the customer. In fact, it is common forthe manufacturers to fix all but one of the system sensitivity-governingparameters prior to sale. Usually, the one parameter which remainsadjustable is the gain of the tuned preamplifier which receives thereflected energy waves. By leaving this parameter adjustable, theultimate customer or installer can adjust the sensitivity to an optimumlevel which, in turn, is governed by the environmental noise level. Asindicated above, however, the noise level is difficult to ascertainwithout the use of rather expensive test equipment.

In accordance with the present invention, means are provided forfacilitating the proper adjustment of system sensitivity. Such meansincludes an environmental test switch 40 which, when activated, servesto increase the system sensitivity, via circuitry to be described, by apredesired factor, which represents a predesired margin of safetyagainst false alarms. Preferably, when switch 40 is on or in an activeposition, the effective bandpass of filter 31 is widened, the timeconstant of integrator 33 is reduced, and the threshhold of detector 34is reduced. By appropriately adjusting these parameters, the combinedeffect is the predesired increase in system sensitivity.

Upon activating switch 40, thereby increasing the sensitivity by adesired factor, the installer then adjusts the gain of tunedpreamplifier 23 to a level just under that where the alarm relay istripped or activated by the background noise. This level can be achievedby merely increasing the preamplifier gain until alarm activation isproduced, and then gradually decreasing the gain until no alarm isproduced. When the environmental test switch is turned off orinactivated, thereby returning the system sensitivity to a level whichis a known factor less than that at which it was set when switch 40 wason," the system sensitivity is properly set for a given margin ofsafety. For the ultrasonic de tection system described herein, a systemsensitivity approximately four times less sensitive than the maximumlevel at which the sensitivity could be set for a given steady statenoise level has been found to provide an adequate margin of safetyagainst false alarming from electrical or acoustical transients in thebackground which exceed such steady state level, without substantiallysacrificing the level of security provided by the system. This safetymargin also prevents false alarms from occurring due to range expansionassociated with changes in temperature and humidity in the protectedarea.

The manner in which switch 40 can be used to increase the sensitivity ofthe system illustrated in FIG. 1 is readily understood with reference toFIGS. 2 and 3. In FIG. 2, typical circuitry for integrator 33 andthreshhold detector 34 is shown schematically. Integrator 33 comprisescapacitor C1 and C2 and resistor R1, and threshhold detector 34comprises resistors RZ-RS and transistor Q1. FIG. 3 illustrates thefrequency response of filter 31 and the alarm tripping threshhold whenswitch S is in on and off positions. When the environmental test switchis off, as in the position shown (shunting terminals A and B), thesystem sensitivity is set at the normal operating level. The timeconstant of integrator 33 is determined by both capacitors Cl and C2,and the alarm activating threshhold of detector 34 (i.e., the voltage V,at which transistor Q1 conducts) is governed by the ratio of R3 to R4.The threshhold voltage required to produce alarm activation when switch40 is off is indicated by V in FIG. 3. At this level, only input signalshaving frequencies within the bandpass F and an amplitude exceeding Vare effective in producing an output from the threshhold detector.

When switch 40 is turned on, capacitor C2 is disconnected from theintegrator circuit, thereby reducing the time constant of integration.Simultaneously, resistor R is connected in parallel with resistor R4thereby reducing the threshhold level of detector 34 to a new level V Asis apparent in FIG. 3, when the threshhold is at V the effectivebandpass of the system is increased to the range F Thus, by merelyturning switch 40 on, the system becomes hypersensitized because 1)input signals of lower amplitude produce an alarm signal due to thereduction of threshhold from V to V (2) input signal frequenciesnormally outside the alarm-tripping bandpass are capable of producing analarm signal due to the widening of the bandpass from F, to F and (3)input signals of shorter duration can produce an alarm signal due to thereduction in the time constant of the integrator. By properly adjustingthe values of the capacitors and resistors, any desired increase insystem sensitivity can be achieved.

As indicated above, ultrasonic intruder detection systems are merelyexemplary of the detection systems wherein the invention has utility. Inaddition, the invention also has utility with electro-optical, microwaveand various other *active" systems, as well as with a variety of"passive" detection systems, including ionization and smoke-sensing firedetection systems and infraredsensing intruder detection systems.

I claim:

1. For use in a detection system for sensing the occurrence of an eventof interest amid a variety of sources of background noise, the systemcomprising means for distinguishing electrical signals characteristic ofthe event of mte rest from electrical signals characteristic ofbackground noise, and means for activating an alarm in response to thesensing of an electrical signal having an amplitude which rises above ordrops below a predefined threshhold level, the improvement comprising,in combination:

switch means for temporarily increasing the level of sensitivity of thesystem by a predesired factor above an initial level; and means forsetting the sensitivity of the system, at a time when said switch meansis effective to produce such a temporary increase in sensitivity, to alevel at which electrical signals characteristic of background noisehave an amplitude substantially equal to said threshold level of theactivating means, whereby the system sensitivity, upon being unaffectedby said switch means, is set at a level which provides a predesiredmargin of safety against alarming due to background noise signals.

2. In an intruder detection system comprising means for transmittingenergy waves into a region wherein intrusion is to be detected; meansfor receiving said trans mitted energy waves upon being reflected and/ormodified by objects within said region and for converting said energywaves into a first electrical signal having an instantaneous amplitudeand frequency proportional to that of the resultant energy wavereceived; detecting means for generating a second electrical signal inresponse to changes in said first electrical signal; integrating meansoperatively coupled to said detecting means for smoothing out rapidvariations in the amplitude of said second electrical signal, the degreeof such smoothing being governed by the time constant of the integratingcircuit, and a threshold sensor operatively coupled to said integratingmeans for activating an alarm when the amplitude of said secondelectrical signal exceeds a predefined threshold level, the improvementcomprising, in combination: switch means operatively coupled to saidintegrating means for temporarily decreasing the time constant of saidintegrating means by a predetermined factor, thereby temporarilyincreasing the sensitivity of the system by a predefined factor; andmeans operatively coupled to said receiving means for adjusting theamplitude of said first electrical signal, at a time when said switchmeans is effective to temporarily increase the system sensitivity andwhen no intruder is present in said region, to a level at whichbackground noise is just effective to produce alarm activation, wherebythe system sensitivity, upon being unaffected by said switch means, isset at a level which provides a predesired margin of safety againstalarming due to signals characteristic of only background noise.

3. In an intruder detection system comprising means for transmittingenergy waves into a region wherein intrusion is to be detected; meansfor receiving said transmitted energy waves upon being reflected and/ormodified by objects within said region and for converting said energywaves into an electrical signal having an instantaneous amplitude andfrequency proportional to that of the resultant energy wave received;thresholdsensing means operatively coupled to said receiving means fortransmitting an alarm activating signal in the event the amplitude ofsaid electrical signal exceeds a predefined threshold level; theimprovement comprising, in combination;

switch means operatively coupled to said thresholdsensing means fortemporarily reducing the threshold level of said threshold-sensing meansby a preat which background noise is just effective to produce alarmactivation, whereby the system sensitivity, upon being unaffected bysaid switch means, is set at a level which provides a predesired marginof safety against alarming due to signals characteristic of onlybackground noise.

1. For use in a detection system for sensing the occurrence of an eventof interest amid a variety of sources of background noise, the systemcomprising means for distinguishing electrical signals characteristic ofthe event of interest from electrical signals characteristic ofbackground noise, and means for activating an alarm in response to thesensing of an electrical signal having an amplitude which rises above ordrops below a predefined threshhold level, the improvement comprising,in combination: switch means for temporarily increasing the level ofsensitivity of the system by a predesired factor above an initial level;and means for setting the sensitivity of the system, at a time when saidswitch means is effective to produce such a temporary increase insensitivity, to a level at which electrical signals characteristic ofbackground noise have an amplitude substantially equal to said thresholdlevel of the activating means, whereby the system sensitivity, uponbeing unaffected by said switch means, is set at a level which providesa predesired margin of safety against alarming due to background noisesignals.
 2. In an intruder detection system comprising means fortransmitting energy waves into a region wherein intrusion is to bedetected; means for receiving said transmitted energy waves upon beingreflected and/or modified by objects within said region and forconverting said energy waves into a first electrical signal having aninstantaneous amplitude and frequency proportional to that of theresultant energy wave received; detecting means for generating a secondelectrical signal in response to changes in said first electricalsignal; integrating means operatively coupled to said detecting meansfor smoothing out rapid variations in the amplitude of said secondelectrical signal, the degree of such smoothing being governed by thetime constant of the integrating circuit, and a threshold sensoroperatively coupled to said integrating means for activating an alarmwhen the amplitude of said second electrical signal exceeds a predefinedthreshold level, the improvement comprising, in combination: switchmeans operatively coupled to said integrating means for temporarilydecreasing the time constant of said integrating means by apredetermined factor, thereby temporarily increasing the sensitivity ofthe system by a predefined factor; and means operatively coupled to saidreceiving means for adjusting the amplitude of said first electricalsignal, at a time when said switch means is effective to temporarilyincrease the system sensitivity and when no intruder is present in saidregion, to a level at which background noise is just effective toproduce alarm activation, whereby the system sensitivity, upon beingunaffected by said switch means, is set at a level which provides apredesired margin of safety against alarming due to signalscharacteristic of only background noise.
 3. In an intruder detectionsystem comprising means for transmitting energy waves into a regionwherein intrusion is to be detected; means for receiving saidtransmitted energy waves upon being reflected and/or modified by objectswithin said region and for converting said energy waves into anelectrical signal having an instantaneous amplitude and frequencyproportional to that of the resultant energy wave received;threshold-sensing means operatively coupled to said receiving means fortransmitting an alarm activating signal in the event the amplitude ofsaid electrical signal exceeds a predefined threshold level; theimprovement comprising, in combination; switch means operatively coupledto said threshold-sensing means for temporarily reducing the thresholdlevel of said threshold-sensing means by a predetermined factor, therebytemporarily increasing the sensitivity of the system by saidpredetermined factor; and means operatively coupled to said receivingmeans for adjusting the amplitude of said electrical signal, at a timewhen said switch means is effective to temporarily increase the systemsensitivity and when no intruder is present in said region, to a levelat which background noise is just effective to produce alarm activation,whereby the system sensitivity, upon being unaffected by said switchmeans, is set at a level which provides a predesired margin of safetyagainst alarming due to signals characteristic of only background noise.